Other

Since trac uses a SQL database to store the data, you can import from other systems by examining the database tables. Just go into ​sqlite command line to look at the tables and import into them from your application.

Using a comma delimited file - CSV

See ​http://trac.edgewall.org/attachment/wiki/TracSynchronize/csv2trac.2.py for details. This approach is particularly useful if one needs to enter a large number of tickets by hand. (note that the ticket type type field, (task etc...) is also needed for this script to work with more recent Trac releases)
Comments on script: The script has an error on line 168, ('Ticket' needs to be 'ticket'). Also, the listed values for severity and priority are swapped.

Using an Excel (.xls) or comma delimited file (.csv)

You can also use it to modify tickets in batch, by saving a report as CSV, editing the CSV file, and re-importing the tickets.

This plugin is very useful when starting a new project: you can import a list of requirements that may have come from meeting notes, list of features, other ticketing systems... It's also great to review the tickets off-line, or to do massive changes to tickets.

Based on the ticket id (or, if no id exists, on the summary) in the imported file, tickets are either created or updated.